difference between 71 72 roadrunner and 73 74 models

I've recently purchased a 73 U code Roadrunner. In looking at Year One, AMD and other sights I'm finding that parts, wiring and body parts are readily available for the 71 72 runners but not so much for the 73 74 runners. reading past articles I got the impression that 71 to 74 all used the new "fuselage" body and that the 73 74 had grille, bumper and tail light/tail panel only changes. Can some one enlighten me about why such a difference in parts and body or door replacement availability?

From the rear they look similar, but there are many changes. Beside what you mentioned, between '72 and '73 the changes include doors, fenders, quarter panels and hood for starters...

thank you so very much. the side view presented clearly shows the body line difference.